Bill Gross, the co-founder and co-chief investment officer of bond giant Pimco, says it is time to write the obituary for stock investing as we know it.

Writing in his August investment letter, the manager of the world's largest bond mutual fund said lower returns on stocks — and bonds, for that matter — means individuals will have to work longer to save for their retirements.

"If financial assets no longer work for you at a rate far and above the rate of true wealth creation, then you must work longer for your money," Gross wrote.

"The cult of equity is dying," he wrote. "Like a once bright green aspen turning to subtle shades of yellow then red in the Colorado fall, investors’ impressions of 'stocks for the long run,' or any run, have mellowed as well."

Gross, whose Pacific Investment Management Co. has $1.82 trillion in assets, took particular issue with the noted economist Jeremy Siegel, who popularized the notion that a portfolio of stocks can return on average 6.6 percent over the long haul.

"The Siegel constant of 6.6 percent real appreciation, therefore, is an historical freak, a mutation likely never to be seen again as far as we mortals are concerned," he said. "Yet the 6.6 percent real return belied a commonsensical flaw much like that of a chain letter or yes – a Ponzi scheme."

Gross' August investment letter is a bit reminiscent of BusinessWeek's famous "Death of Equities" cover story, which appeared in 1979, just before the start of a big bull market.

Gross, whose firm launched its first actively-managed equity mutual fund in 2010 and has former Troubled Asset Relief Program leader Neel Kashkari as its head of global equities, said bonds are no salvation either.

In his April investment letter, Gross struck a similar tone on total return expectations. Gross then said investors should get used to smaller investment returns because of slower global growth and as the financial services industry continues to deleverage, or reduce its reliance on derivatives and borrowed money to generate higher returns.

This time around, Gross said at their currently low interest rates, investors should expect "mere survival" from their bond investments.

"With long Treasurys currently yielding 2.55 percent, it is even more of a stretch to assume that long-term bonds — and the bond market — will replicate the performance of decades past," he wrote.

In his August letter, Gross says the only "magic potion" monetary policymakers have to try and get higher returns for investors is through inflationary policies.

He said inflationary policies might work for bonds, but that they are bad for stocks. And over the long term, Gross said using inflation to solve retirement ills is not a real solution.

"The problem with all of that of course is that inflation doesn't create real wealth and it doesn't fairly distribute its pain and benefits," Gross said.

"The cult of equity may be dying, but the cult of inflation may only have just begun."